The plastic support components for a plastic mailbox are normally secured by releasable fasteners to an upright post such as a four by four pressure treated wood post. Such posts are sturdy, resist decay and are reasonable in cost; however, they often have imperfections in dimension due to warpage and rough cutting. The interior pocket or pockets of the plastic support post must be made sufficiently large to accommodate the deviations in post dimensions; however, this can result in a loose fit for some installations. Even though the plastic parts of the support are secured by fasteners to the wood post, the lack of support contact otherwise between the post and the plastic support components may allow the mailbox to move, thereby producing undesired instability. Some prior constructed plastic mailboxes use a plastic support, which confronts three of the four sides of a square section wooden post with one side of the plastic support secured to the post. A separate piece of flat plastic is provided for attachment to the fourth side to improve the appearance of the mailbox with attendant increase in cost.